Use the ISNUMBER function to check if a value is a number. ISNA will return TRUE when value is numeric and FALSE when not.
For example, =ISNUMBER(A1) will return TRUE if A1 contains a number or a formula that returns a numeric value. If A1 contains text, ISNUMBER will return FALSE.
Normally, value is supplied as a cell address.
ISNUMBER is part of a group of functions called the IS functions.